I. Field
The present invention relates to a communication system. More particularly, the present invention relates to a novel and improved method and apparatus for forward power control in a communication system.
II. Background
A modern day communication system is required to support a variety of applications. One such communication system is a code division multiple access (CDMA) system which conforms to the “TIA/EIA/IS-95 Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System,” hereinafter referred to as the IS-95 standard. The CDMA system allows for voice and data communications between users over a terrestrial link. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled “SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS,” and U.S. Pat. No. 5,103,459, entitled “SYSTEM AND METHOD FOR GENERATING WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM,” both assigned to the assignee of the present invention and incorporated by reference herein.
In a CDMA system, communications between users are conducted through either one or more Access Networks or via a data network for data applications. An Access Network comprises a plurality of Access Points. In one embodiment, the data network is the Internet. It would be understood by those skilled in the art that the data network could be any kind of data network known in the art. A first Access Terminal may communicate to a second Access Terminal by transmitting data on a reverse link to an Access Network or a data network.
When the data is transmitted to the Access Network, the Access Network receives the data and can route the data on the forward link to the second Access Terminal or can route the data to another Access Network. The forward link refers to transmission from the Access Network to an Access Terminal and the reverse link refers to transmission from the Access Terminal to an Access Network. In IS-95 systems, the forward link and the reverse link are allocated separate frequencies.
The Access Terminal calculates a signal-to-noise-and-interference ratio C/I for a received forward link signal. The Access Terminal's calculated C/I determines the information rate that can be supported for the forward link from the Access Point to a user's Access Terminal. That is, a given level of performance for the forward link is achieved at a corresponding level of C/I. A method and apparatus for selecting an information rate is disclosed in U.S. patent application Ser. No. 08/963,386 entitled “METHOD AND APPARATUS FOR HIGH RATE PACKET TRANSMISSION,” filed Nov. 3, 1997, now U.S. Pat. No. 6,574,211 issued Jun. 3, 2002 to Padovani et al., which is assigned to the assignee of the present invention and fully incorporated herein by reference.
The power at which an Access Point transmits data to an Access Terminal is called the forward link transmit power. The forward link transmit power is at a level required for transmitting data over the forward link reliably. The forward link transmit power is often more than is required for a given reliable data rate. This overage is called “quantization loss.” Quantization loss is the quantity of transmit power on the forward link that is beyond that required for a given reliable data rate and therefore is a lost quantity of transmit power, i.e., is wasted. Quantization loss is a problem because it is excess transmit power that limits forward link efficiency and throughput. Excess transmit power of an Access Point causes interference for Access Terminals being served by adjacent Access Points. This interference causes the Access Terminal being served by the Access Point to observe a lower C/I and consequently have a lower data rate. Thus, throughput is limited.
Decreasing the quantization loss would result in a gain in forward link throughput efficiency and throughput. Therefore, a system and method that decreases the loss due to excess transmit power is desired.
The parameters that measure the quality and effectiveness of a data communication system are the transmission delay required for transferring a data packet and the average throughput rate of the system. Transmission delay does not have the same impact in data communication as it does for voice communication, but it is an important metric for measuring the quality of the data communication system. The average throughput rate is a measure of the efficiency of the data transmission capability of the communication system.
When an Access Terminal is in an interference-limited location, i.e., on a cell boundary, the Access Terminal can receive pilot signals from multiple Access Points interfere with the pilot signal from the Access Point that is serving the Access Terminal. Consequently, the C/I observed by the Access Terminal is lower on the cell boundary than when the Access Terminal is not on a cell boundary. As a result, the Access Terminal has a lower served rate and a lower data rate than when the Access Terminal is not on a cell boundary. The served rate is the rate at which an Access Point schedules the Access Terminal for service. The data rate is the rate at which the Access Point sends forward link data to the Access Terminal.
From a service point of view, assuming Access Terminals are served by the same Access Point, the Access Terminals that are on a cell boundary are getting served slower (i.e., higher transmission delay) and at a slower data rate (i.e., average throughput rate) than Access Terminals that are not on a cell boundary. A system and method that services more users in a period of time, and quickly services those users, is desired.